1. Field of the Invention
The invention relates generally to the field of wellbore construction fluid testing. More specifically, the invention relates to apparatus and methods for testing properties of various fluids used during wellbore construction.
2. Background Art
During the drilling of a wellbore through subsurface rock formations, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. The drilling fluid may act to, among other functions, remove drill cuttings from the bottom of the wellbore to the surface during fluid circulation, suspend drill cuttings and weighting material when fluid circulation is interrupted, control subsurface pressures, maintain the integrity of the wellbore until the exposed portion of the wellbore is cased and cemented, isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, cool and lubricate the drill string and bit, and/or to maximize penetration rate.
In most rotary drilling procedures the drilling fluid takes the form of a “mud,” a term of art used to mean a liquid having solids suspended therein. The characteristics of the liquid are selected to and the solids function to impart desired rheological properties to the mud and in certain cases to increase the density thereof in order to provide a suitable hydrostatic pressure at the bottom of the well.
The drilling mud may be either a water-based or an oil-based mud. Drilling mud may consist of, for example, polymers, biopolymers, clays and organic colloids added to a water-based fluid to obtain the required viscosity and filtration properties. High density minerals, such as barite or calcium carbonate, may be added to increase density. Solids from the formation are incorporated into the mud and often become dispersed in the mud as a consequence of drilling. Further, drilling muds may contain one or more natural and/or synthetic polymeric additives, including polymeric additives that increase the rheological properties (e.g., plastic viscosity, yield point, gel strength) of the drilling mud, and polymeric thinners and flocculents. Polymeric additives included in the drilling fluid may act as fluid loss control agents. Fluid loss control agents, such as starch, prevent the loss of fluid to the surrounding formation by reducing the permeability of filter cakes formed on the newly exposed rock surface. In addition, polymeric additives are employed to impart sufficient carrying capacity and thixotropy to the mud to enable the mud to transport the cuttings up to the surface and to prevent the cuttings from settling out of the mud when circulation is interrupted.
International Patent Application Publication No. WO 2008/112795, the underlying patent application for which is owned by the assignee of the present invention, describes a device for testing drilling mud to ensure that the fluid properties are acceptable to the user. An apparatus for testing a drilling fluid as described in the foregoing publication includes a vessel having a fluid inlet, a fluid outlet, and a pair of opposed impermeable platens disposed within the vessel. The apparatus further includes a test fluid container in fluid communication with the fluid inlet, and a collection container in fluid communication with the fluid outlet. Additionally, the disclosed system includes a data acquisition device configured to receive data from at least one of the vessel, the test fluid container, and the collection container.
Another apparatus for testing drilling fluids is described in International Patent Application No. WO 2008/058253, the underlying patent application for which is also owned by the assignee of the present invention. An apparatus described in the foregoing publication includes a vessel having a fluid inlet, a filtrate outlet, a fluid outlet, and at least one permeable medium disposed within the vessel. The system further includes a base fluid container in fluid communication with the fluid inlet, a test fluid container in fluid communication with the fluid inlet, a filtrate container in fluid communication with the filtrate outlet, and a collection container in fluid communication with the fluid outlet. Additionally, the system includes a data acquisition device configured to receive data from at least one of the vessel, the fluid container, the filtrate container, and the collection container.
As described in the foregoing publications, effective fluid loss control is highly desirable to prevent damaging the formation in, for example, completion, drilling, drill-in, displacement, hydraulic fracturing, work-over, packer fluid emplacement or maintenance, well treating, or testing operations. In certain drilling environments, the formation may be exceptionally prone to damage from fluid loss. Examples of such drilling operations may include depleted zone drilling. Depleted drilling zones may be especially prone to fractures (i.e., cracks and disruptions in a formation that may be either naturally formed or induced) Fracturing during the drilling operation, also known as induced fracturing, typically occurs in permeable rocks such as sandstone and carbonates or within impermeable rock typified by shale formations. Induced fracturing is of particular concern when drilling into depleted zones where a drop in pore pressure is anticipated as the reserves decline. In such situations, drilling then becomes more of a technical challenge as the mud weight required to support a section may exceed the tensile strength, or fracture resistance, of the formation. This in turn could lead to increased drilling fluid losses and increased well costs.
One technique under development for drilling in fracture susceptible formations is to dispose a gellable fluid in the wellbore such that it will enter fractures in susceptible formations in liquid form and then undergo state change to a gel. If the liquid state and gel properties are suitable for the particular formation, the fluid will act to seal the fractures and to reduce the incidence of such fractures propagating as drilling resumes, as well as to reduce the incidence of fluid being returned to the wellbore from fractures as they close upon reduction in hydrodynamic pressure when mud circulation is interrupted.
Properties and example compositions of such gellable liquids and test results of using such gellable liquids are described, for example, in Mark S. Aston, et al., A New Treatment for Wellbore Strengthening in Shale, paper no. 110713, Society of Petroleum Engineers, Richardson, Tex. prepared for presentation at the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, Calif., Nov. 11-14, 2007.
It is desirable to have an apparatus and method to test fluid properties, in particular gellable liquids, to confirm, for example, their fracture sealing and related mechanical properties (e.g., fracture pressure and compressive strength). While the apparatus disclosed in the two above cited International Patent Application publications are well suited for testing fluid loss and related properties of drilling fluids, they have not proven very useful for testing gellable fluids after the gel has set or cured. In particular, the foregoing described apparatus may be difficult to clean after gel set, and neither apparatus has any features for testing compressive strength or fracture pressure of a set gel. There continues to be a need for an apparatus and method to test properties of various wellbore construction fluids.